Application Ser. No. 07/356,375 relates to commutation control methods and circuitry for switched reluctance electric motors. In a plural phase switched reluctance electric motor, the phases are energized in cyclic sequence, generally one phase at a time, though some overlap may be desirable in certain applications. The cyclic sequence of energization of the phases creates a rotating magnetic field producing a torque on the rotor which then rotates to follow the rotating magnetic field. Generally, when the rotor reaches a maximum alignment position relative to the presently energized phase, the next phase should then be energized, to maximize torque production. The change from one motor state energizing one phase to the next motor state energizing the next phase is called commutation. It is desirable to know the position of the rotor, in order to select the proper commutation timing and phase energization, and hence provide the optimum magnetic field pattern for producing optimum torque on the rotor.
Various commutation methods are known in the prior art for commutating to the next motor state to energize the next phase. Various external sensors are known for sensing rotor position. A drawback of external sensors is that they add cost to the motor control, and require additional wiring.
Various sensorless motor control systems are known in the prior art. These systems sense a given motor parameter, such as inductance as measured by back EMF, a frequency change in switching because of an inductance change, a test pulse current in an nonenergized winding, and so on. A drawback of these methods is that they are motor-dependent, i.e. inductance varies from motor to motor and with differing sizes, manufacturers, and construction methods. Another drawback is that such methods only provide a general reference point for where the motor is in its rotation.
The noted '375 application provides a control method which is motor-independent, and which senses an actual change in motor operation, not just a change in a motor parameter. The '375 application further enables an exact position reference for the motor. In the '375 application, regeneration current is sensed in the energized phase of a switched reluctance electric motor. In the preferred embodiment, individual phase regeneration current is sensed. In an alternate embodiment, bus regeneration current is sensed.
Application Ser. No. 07/389,874 relates to commutation control methods and circuitry for electric motors including at least three phases energized in a cyclic sequence having a plurality of states, wherein during each state, two of the phases are energized and the third phase is unenergized. In a plural phase electric motor, the phases are energized to create a rotating magnetic field, which the rotor will follow due to the torque produced thereon. To sustain rotation of the rotor, the phase windings are energized in a given sequence having a plurality of states. The change from one state to the next state is called commutation. Commutation ensures continued rotation of the magnetic field, and hence continued rotation of the rotor. It is desirable to know the position of the rotor, in order to select the proper commutation timing and energization state of the various windings, and hence provide the optimum magnetic field pattern for producing optimum torque on the rotor. When the rotor passes a given position, it is desired to commutate to the next state in the energization sequence of the phase windings, to continue to apply torque to the rotor.
In the control of variable speed electric motors, it is desirable to have feedback of motor speed. This feedback typically consists of an external sensor that connects to the motor shaft and directly reads the rotor information. This external sensor adds an undesirable cost to the motor control, and requires additional wiring to the motor. The burden of these extra wires is compounded in applications such as air conditioning compressors, where the motor is hermetically sealed. It is therefore desired in various applications to eliminate external sensors and extra wires.
Various sensorless motor control systems are known in the prior art. In one method, back EMF of an unenergized phase is sensed to control commutation to the next state. In another method, changing reluctance of the unenergized phase is sensed as a function of rotor position, to control commutation to the next state. In the latter method, the unenergized phase winding is pulsed with a test current to determine changing reluctance.
In the noted '874 application, regeneration current is sensed from the unenergized phase, and the motor is commutated to the next state in response to such regeneration current. In the preferred embodiment, bus regeneration current is sensed with all commutation switches off. In an alternate embodiment, individual phase regeneration current is sensed.
Application Ser. No. 07/400,460 relates to commutation control methods and circuitry for electric motors including at least three phases energized in a cyclic sequence having a plurality of states, wherein during each state, two of the phases are energized and the third phase is unenergized. In the noted '460 application, a method is provided for regulating excessive current in a phase of the motor when switching from one pair of phases to the next pair of phases, i.e. when commutating from one state to the next state.
In one type of motor drive, it is desirable to turn on one of the power switches and to pulse width modulate the other power switch, corresponding to the phases being driven, to regulate current in the pair of phases. This method of current regulation allows the motor phase current to flow through the switch that is held on and either through the switch that is being pulse width modulated or through a flywheel diode. This produces a lower deviation in phase current for a given pulse width modulation frequency than if both switches were pulse width modulated. A problem with this type of current control, however, is that the current flowing through the diode does not appear on the bus current sensor, and it is undesirable to have a current sensor in each of the motor phases to read such current because of the added cost. Another problem occurs upon commutation because if the current in the newly energized phase builds up faster than the current dissipates in the previously energized phase, a current overshoot or spike will occur. This current spike can damage the motor's magnets and power switches.
One manner known in the prior art to eliminate or at least minimize the noted potentially destructive current spikes is to provide a commutation delay interval between commutations from state to state, during which delay interval all phases are unenergized, to minimize current spikes upon commutation. The duration of the delay interval is chosen to balance the trade-off between a maximum delay affording the best protection against current spikes versus a minimum delay which is desired to provide maximum energization current to the phases. If the power switches are held off too long, particularly at high motor speeds, there is not sufficient time to build up current in the newly energized phases, and motor efficiency and power suffers.
The invention in the '460 application overcomes the noted trade-off of a fixed delay interval and optimizes the delay interval for different operating conditions of the motor, to provide maximum delay when needed to eliminate current spikes, and to provide minimum delay when unneeded, to in turn provide maximum energization current to the phases. The invention in the '460 application provides a variable commutation delay interval whose duration is varied according to a given motor operating parameter during operation of the motor, to provide the noted optimization.
Application Ser. No. 07/403,308 relates to current regulation schemes for energizing electric motors, and more particularly to PWM (pulse width modulated) current control regulation. The '308 application provides a method and system for regulating the average current of a PWM current waveform used in a motor drive without restricting the bandwidth by filtering the current feedback signal. This is accomplished by setting the peak current level for the motor and recalculating same every PWM cycle.
Current regulation schemes in the prior art generally use either peak current regulation or time averaged current regulation. Peak current regulation controls the peak current without regard for the average current, which results in a drop in average current and motor torque as speed increases. Time averaged current regulation uses a highly filtered current signal to obtain the average current, which severely limits the frequency response of the current loop and degrades the overall response time of the drive. The invention of the '308 application overcomes these disadvantages and maintains full available motor torque at all speeds without limiting the frequency response of the current loop.
To maintain constant torque as motor speed changes, it is necessary to regulate the average current in the motor. As noted above, in the prior art this was accomplished or approximated by either time averaging (filtering) of the phase current waveform, or by peak current regulation. When time averaging is used, individual current sensors in each motor phase or PWM operation of both power switches in each switch pair is necessary to continuously monitor the motor current. Each of these methods has a cost and/or performance limitation when applied to a motor drive.
Individual current sensors in each motor leg allows continuous measurement of the motor phase currents. When this is used in the prior art, the phase current is time averaged and compared to a reference to allow regulation of average current. This method has the cost disadvantage of requiring three isolated current sensors, and the performance disadvantage of increasing the response time of the control due to the time averaging of the current waveform.
Peak current regulation as used in the prior art limits the peak current of each PWM cycle to a fixed level. This method has the advantages of using only one current sensor and having a faster response time since the current waveform is not time averaged. However, this method has the disadvantage of reduced torque at high motor speeds. This is caused by the back EMF of the motor which increases as speed increases and causes increased ripple in the current waveform. Different motor inductances will also cause different amounts of ripple and different average currents. If the peak current is held constant, the average current decreases. This decrease in average current results in proportionately less torque.
PWM switching of both switches instead of one switch forces the motor phase current to flow through the DC bus sensor. While this allows measurement of all phase currents using one current sensor, in the prior art the phase current is time averaged or peak regulated resulting in the performance limitations noted above. Further disadvantages are increased losses in the solid state switches, increased losses in the motor due to higher ripple in the current waveform, and increased motor noise. In the '308 application, average current is maintained at a fixed level regardless of motor speed and inductance without adding delays by time averaging the current waveform or requiring more than one current sensor.
The present invention provides AC power line current regeneration. An AC power line is energized from a DC power source through pulse width modulated switch means to apply energization voltage to the AC power line in an on condition of the switch means until line regeneration current reaches a given peak value, and then turning off the switch means such that line regeneration current decreases, and then turning the switch means back on until line regeneration current again increases to the peak value, and then turning off the switch means, and so on, to regulate line regeneration current, each pair of on and off conditions of the switch means constituting a PWM cycle. The required peak line regeneration current value from a previous PWM cycle is sampled, and the required peak line regeneration current value for the present PWM cycle is calculated, to provide high speed fast response PWM line regeneration current control regulation. The preferred method involves sampling the required peak line regeneration current value from a previous PWM cycle, I(peak last), sampling line regeneration current value at turn-on of the switch means, I(turn-on), and calculating the required peak line regeneration current value for the present PWM cycle, I(peak present), from both I(peak last) and I(turn-on).